1. Field of the Invention
The present invention relates to a reflection type liquid crystal display and a method for manufacturing the same, and more specifically, to a reflection type liquid crystal display having a plurality of aligned micro lens and a method for manufacturing the same.
2. Description of the Related Art
In an information-oriented society these days, the role of an electronic display is becoming more important. All kinds of electronic displays are widely used in various industrial fields. As techniques of the electronic display field are continuously developed, various electronic displays having new functions are provided corresponding to diverse requirements of the information society.
Generally, electronic display device is an apparatus for visually transmitting information to a person. That is, an electronic display device can be defined as an electronic apparatus, which converts an electrical information signal output from various electronic equipment into a visually recognizable optical information signal. Also, it may be defined as an electronic apparatus serving as a bridge for connecting the person and the electronic equipment.
These electronic displays are classified into an emissive display in which the optical information signal is displayed by a light-emitting method, and a non-emissive display in which the signal is displayed by an optical modulation method such as light-reflecting, dispersing and interference phenomena, etc. As the emissive display called an active display, for example, there are a CRT (Cathode Ray Tube), a PDP (Plasma display panel), an LED (Light emitting diode) and an ELD (Eelectroluminescent Display), etc. The non-emissive display is called a passive display, an LCD (Liquid Crystal Display) and an EPID (Eelectrophoretic Image Display), etc fall in that category.
The CRT has been used in an image display such as a television receiver and a monitor, etc,, over the longest period of time. The CRT has the biggest market share because of its high displaying quality and low price, but also has many disadvantages such as heavy weight, large volume and high power consumption.
Meanwhile, as various kinds of electronic devices are small in size and lighter in weight and use lower voltage and less power in driving the electronic devices due to rapid advancement of semiconductor technologies, demands have been increased for a flat panel type display being slimmer and lighter property as well as lower driving voltage and consuming less power.
Among flat panel type displays, the LCD is much slimmer and lighter than any other displays and it requires lower driving voltage and less power consumption. Also, the LCD has the displaying quality similar to the CRT. Therefore, the LCD is widely used in various electronic devices. Further, since the LCD can be manufactured relatively easily, its application area becomes wider.
The LCD is classified into a transmissive type LCD, which displays an image using an external light source and a reflection type LCD, which displays the image using ambient lights instead of the external light source.
The reflection type LCD has an advantage because it consumes less power and shows an excellent display at outdoor compared to the projection type LCD. Further, the reflection type LCD is thin and light because an additional light source such as backlight device is not required.
However, the current reflection type LCD shows darker image than its competition and fails to show a high resolution and multicolor images. Therefore, the reflection type LCDs are restrictively used for a product that requires a simple pattern display, such as, numbers or simple characters.
To use a reflection type LCD for various electronic displays, a high resolution and a multicolor display together with an enhanced reflection luminance are necessary. In addition, a proper brightness, rapid response time and higher contrast are necessary.
In current reflection type, LCDs, two technologies are combined to enhance the brightness. One is enhancing the reflection efficiency of the reflective electrode, and the other is achieving an ultra high aperture ratio.
There is disclosed a method for enhancing the reflection efficiency by forming bumps to a reflective electrode in U.S. Pat. No. 5,610,741 (issued to Naofumi Kimur) entitled “Reflection type liquid crystal display device with bumps on the reflector.”
FIG. 1 is a partial plan view of the reflection type LCD device provided in the '741 U.S. Pat. No. and FIG. 2 is a sectional view of the reflection type LCD device of FIG. 1.
Referring to FIG. 1 and FIG. 2, the reflection type LCD device has a first substrate 10, a second substrate 15 disposed facing the first substrate 10 and a liquid crystal layer 20 interposed between the first substrate 10 and the second substrate 15.
The first substrate 10 includes a first insulating substrate 30 on which a plurality of gate bus wirings 25 are formed. Gate electrodes 35 branch off from the gate bus wirings 25. Additionally a plurality of source bus wirings 40 are provided so as to cross the gate bus wirings 25. The source bus wirings are insulated from the plurality of gate bus wirings 25 by means of an insulating layer. Source electrodes 45 branch off from the source bus wirings 40.
Reflective electrodes 50 are formed between the first substrate 10 and the liquid crystal layer 20 and are disposed in a plurality of rectangular regions formed by crossing the plurality of gate bus wirings 25 and the plurality of source bus wirings 40.
The reflective electrode 50 is connected with thin film transistor (TFT) 55 formed on the first substrate 10. The TFT 55 serves as a switching device with the gate bus wiring 25 and the source bus wiring 40.
A plurality of dents 70 and 71 are provided on the surface of the reflective electrode 50, making the surface rugged.
The plurality of dents 70 and 71 are irregularly arranged on the entire surface. The reflective electrode 60 and a drain electrode of the TFT device 55 are connected to each other through a contact hole 65.
The gate bus wiring 25 and the gate electrode 35 are formed on the first insulating substrate 30 made of, for example, glass by depositing tantalum (Ta) film using a sputtering method and patterning the deposited Ta film using a photolithography method.
Next, the gate insulating film 75 is formed to cover the gate bus wiring 25 and the gate electrode 35. The insulating film 75 is formed, for example, to a thickness of 4000 Å. by depositing a SiNx film through a plasma CVD (Chemical Vapor Deposition) method.
A semiconductor layer 80 of amorphous silicon (a—Si) is formed on the gate insulating film 75 over the gate electrode 35. Contact layers 85 and 90 of n+ type impurity-doped a—Si layer are formed on the semiconductor layer 80.
Subsequently, molybdenum (Mo) film is formed on the first insulating substrate 30 to cover the resultant structure formed in the above-mentioned manner and then the Mo film is patterned to form a source bus wiring 40, a source electrode 45 and drain electrode 60. In such a manner, TFT 55 is manufactured.
On the entire surface of the insulating substrate 30 in which the TFT device 55 was formed are formed an organic insulating film 95 and a reflective electrode 50 each having a rugged surface.
FIGS. 3A, 3B and 3C are sectional views showing the steps of forming the organic insulating film and the reflective electrode in the device shown in FIG. 2.
Referring to FIG. 3A, a resist film 100 is formed on the surface of the first insulating substrate 30 to cover the TFT device 55 by a spin coating method. After that, the resist film 100 is pre-baked.
Next, a 110 where a light transmitting region 105 and a light shielding region 106 are formed in a predetermined pattern is arranged over the applied resist film 100 and exposure and development treatments are carried out. Thereby, bumps 115 corresponding to the pattern of the 110 are formed. Thermal treatment to such a substrate is carried out, whereby a bump 115 whose angles are rounded off is formed as shown in FIG. 3B.
Referring to FIG. 3C, an organic insulating film 95 is applied to cover the bumps 115, for example, by the spin coating method and thereby the surface of the formed organic insulating film 95 becomes rugged due to the bump 115.
Subsequently, the inorganic insulating film 95 is patterned using a mask (not shown) to form a contact hole 65 exposing a surface of the drain electrode 60 of the TFT device 55. A metal film of aluminum (Al) or nickel (Ni) as a reflective electrode 50 is formed on the organic insulating film 95. At this time, the contact hole 65 is filled with the reflective electrode material. The reflective electrode material is formed by the vacuum sputtering method. As a result, dents 70 and 71 are formed on the surface of the reflective electrode 50 such that they have shapes corresponding to those of the organic insulating film 95.
Returning to FIG. 2 again, a first orientation film 120 is formed on the reflective electrode 50 and the inorganic insulating film 95, whereby the first substrate 10 is completed.
The second substrate 15 includes a second insulating substrate 140 on which a color filter 125, a common electrode 130 and a second orientation film 135 are formed.
The second insulating substrate 140 is comprised of glass. A color filter 125 corresponding to each of pixels 145 and 146 is formed on the second insulating substrate 140. On the color filter 125, is formed a common electrode 130 of a transparent material such as ITO (Indium tin oxide), etc., and on the common electrode 130 is formed a second orientation film 135. These elements make the second substrate 15.
The second substrate 15 is aligned over the first substrate such that the second substrate 15 faces the first substrate 10. Then, a liquid crystal layer 20 including liquid crystal and pigment is injected into a space between the first substrate 10 and the second substrate 15 using a vacuum injection method, thereby completing a reflection type LCD.
However, although the conventional LCD enhances the reflection efficiency by forming such a plurality of dents at the reflective electrode, it has some problems as follows:
First, the conventional reflection type LCD has hemispherically shaped dents serving as micro lenses and having different sizes in order to enhance the reflection efficiency but the ridge portions where the dents are not formed have different sizes depending on their positions rendering the reflection efficiency non-uniform. In other words, different sizes of the ridge portions as well as different heights in a region having different sizes of the dents cause different reflection ratios depending on the regions. This makes the reflection ratio of the reflective electrode non-uniform. Thus, the lowering of the reflection uniformity in the reflective electrode results in the uniformity in the orientation of liquid crystal substance, which acts as a factor lowering the contrast of an image displayed on an LCD. Also, there is a high probability that the non-uniformity in the orientation of liquid crystal substance induces fog failure as well as light leakage and afterimage.
In addition, the conventional reflection type LD has a drawback because different sizes of the dents and different sizes of the regions between the dents makes it difficult to precisely control the sizes of the dents and the space between the dents to meet the specification in real world.
Furthermore, although the dents of different sizes overlap each other, the semispherical shapes of dents make it difficult to completely block a scattered reflection of an incident light at the dents portion and thus it is limited to enhance the image quality.
Moreover, since the conventional reflection type LCD has a regular quadrilateral pixel shape, it has drawbacks in that not only a design should be performed from the start point so as to apply it to displays which request respective different pixel sizes and alteration of pixel sizes depending on the variety of information telecommunication apparatus such as hand-held terminals or liquid crystal television receivers but also a process condition should be newly secured. Especially, it is very difficult to apply it to electronic displays such as a mobile phone requesting to show a high reflectivity along a specific direction.